Gas laser with adjustable mirror

ABSTRACT

A gas laser having an elongated plasma tube with a fixed mirror at one end and an adjustable mirror at the other end. An isolation element such as a Brewster window is disposed in the plasma tube near the adjustable mirror to create a gastight discharge compartment between itself and the fixed mirror while maintaining the regenerative path defined by and extending between the mirrors. An active laser gas is disposed in the discharge compartment and appropriate means for exciting the gas to a lasing state is provided whereby alignment of the mirrors may be accomplished while the laser is in operation.

United States Patent.

[72] Inventors William P. Kolb, Jr.

Manhattan Beach; Dale E. Crane, Torrance, both of Calif.

[21] Appl. No. 869,632

[22] Filed Oct. 27, 1969 [45] Patented Sept. 28, 1971 [73] AssigneeHughes Aircraft Company Culver City, Calif.

[54] GAS LASER WITH ADJUSTABLE MIRROR 1 Claim, 2 Drawing Figs.

[52] U.S. Cl 331/945, 3 13/217 [51] lnt.Cl 1101s 3/02 [50] FieldofSearch 331/945; 313/217 56] References Cited UNITED STATES PATENTS3,187,270 6/1965 Kogelnik 331/945 3,396,343 8/1968 Wesselink 331/9453,435,363 3/1969 Patel 331/945 3,492,599 1/1970 Rigrod 331/945 PrimaryExaminer-Ronald L. Wibert Assistant Examiner-R. J. WebsterAttorneys-Paul M. Cable and W. H. MacAllister, Jr.

ABSTRACT: A gas laser having an elongated plasma tube with a fixedmirror at one end and an adjustable mirror at the other end. Anisolation element such as a Brewster window is disposed in the plasmatube near the adjustable mirror to create a gastight dischargecompartment between itself and the fixed mirror while maintaining theregenerative path defined by and extending between the mirrors. Anactive laser gas is disposed in the discharge compartment andappropriate means for exciting the gas to a lasing state is providedwhereby alignment of the mirrors may be accomplished while the laser isin operation.

PATENTED SEP28 IQYI GAS LASER WITH ADJUSTABLE MIRROR Gas lasers can beconstructed generally in two different fashions, each having distinctadvantages and disadvantages. In the first category of construction issimply a plasma tube bottle in which windows (usually Brewster anglewindows) are attached to the vacuum envelope and in which theregenerative cavity minors are supported externally of the bottle. Thesecond construction is one in which the laser mirrors are attacheddirectly to the vacuum bottle so that the mirrors complete the vacuumenvelope.

In the first example, the alignment of the external mirrors is criticalboth with respect to each other and with respect to the plasma tubebore. In this case, a cavity structure must be provided to hold themirrors in place with respect to one another and with respect to thetube. The Brewster windows serve to provide very low loss in one planeof polarization and relatively high loss in the orthogonal plane ofpolarization. The laser will then oscillate in only the low loss planeof polarization and the output will be plane polarized or linear. Thistype of com struction allows mirror alignment while the laser is inoperation and a predicted linear polarization of the output beam isprovided but it has a poor cavity stability with respect to temperatureand vibration and is especially prone to contamination from dust,humidity, etc.

On the other hand, the directly attached mirror configuration, aspreviously described, has prealigned mirrors that never need adjustmentor cleaning. The cavity structure is the vacuum envelope itself, whichin general is fabricated of material that is relatively stable withtemperature variations. This internal mirror construction creates nopreferred polarization in the laser output and adjacent longitudinalcavity modes may tend to oscillate in orthogonal planes of polarizationto result in an operation wherein the output laser beam polarizationcannot be predicted and may even change with time. Although the internalmirror-type tube has good built-in stability with respect to temperatureand vibration, the mirrors are fixed in place and require alignmentprior to laser oscillation which is difficult to accomplish and leads toa high rate of rejection at the quality control station of a productionline.

Still another construction technique has been used which overcomes thedisadvantage of not being able to predict the output beam polarizationbut still requires the necessity of prealignment of the mirrors andfurther introduces yet another disadvantage. This technique essentiallyamounts to replacement of one of the mirrors with a mirrored half prismarrangement wherein the entrance surface is set at Brewster's angle toprovide low loss in one plane of polarization. The incoming wave is thenrefracted to the mirrored rear prism surface such that the wave willthen reflect back on itself, thereby retracing its path and forming atuned oscillator cavity. This system also serves the purpose ofselecting wavelengths since the refractive index of the prism varieswith wavelength. Detracting from this above-described advantage is thepreviously mentioned fact that the mirrors must be prealigned, and thenew problem presented is that the device is subject to mirrormisalignment caused by temperature variations since the half-prismarrangement is highly sensitive to temperature detuning. in fact, for along radius mirror cavity, this detuning is sufficient to stop laseroscillation where the temperature changes only a few degrees centigrade,while the problem is not nearly so severe in the near hemisphericalcavity due to decreased mirror alignment sensitivity, it can causeunwanted power variations as the temperature varies.

From the foregoing, it should be clear that a device incorporating thebest features of each type of construction without introducing newundesirable features or adding significantly to the cost of manufacturewould be a significant advancement of the gas laser art.

Accordingly, it is an object of the present invention to provide a gaslaser which is not subject to the disadvantages of the prior art andwhich incorporates an adjustable mirror for ease of mirror alignmentwhile the laser is in operation.

It is another object of the present invention to provide a simple, lowcost, stable, and maintenance-free gas laser.

It is still another object of the present invention to provide a gaslaser in which maximum power output may be attained and in whichlinearly polarized output is available.

it is yet another object of the present invention to provide highquality gas lasers at costs comparable to lower quality internal mirrorlasers presently on the market.

Still another object of the present invention is to provide a gas laserin which undesired competing modes of oscillation may be discouraged.

These and other objects of the invention are obtained in a gas laserincluding an elongated plasma tube having first and second ends andregenerative cavity means including a fixed mirror at the first end andan adjustable mirror at the second end for defining a laser regenerativepath between the mirrors through the plasma tube. Isolation means isdisposed in the plasma tube and spaced from the adjustable mirror forproviding a gastight discharge compartment between itself and the firstend while maintaining the regenerative path. Also an active laser gas isdisposed in the discharge compartment of the laser tube and excitationmeans is coupled to the active laser gas for exciting the gas to alasing state.

The isolation means may take the form of a Brewster angle window, or anoptically transparent plate may be disposed orthogonally with respect tothe regenerative path where this type of internally provided means forpolarizing the laser energy is not desired.

The invention and specific embodiments thereof will be describedhereinafter by way of example and with reference to the accompanyingdrawing wherein like reference numerals refer to like elements or partsand in which:

FIG. 1 is a cross-sectional view of a preferred embodiment of thepresent invention; and

FIG. 2 is a cross-sectional view of a portion of another embodiment ofthe invention.

Referring now to the drawing and more particularly to the gas laser 11of FIG. 1, there is shown an elongated plasma tube 13 having arelatively larger diameter main body envelope portion 15 and arelatively smaller diameter anode envelope portion 17. The tube 13 alsohas a first end 19 whereat a capillary-supporting hollow tubulation 21and a coaxially disposed output mirror-supporting portion 23, and asecond end 25 with an adjustable mirror-supporting portion 27. Themirrorsupporting portions are shown having a ball socket surface so thatan output, partially transmissive mirror 29 and a highly reflectingadjustable mirror 31 may be moved for alignment purposes; the formerbeing aligned prior to operation, while the latter being aligned whilethe laser is oscillating.

Within the plasma tube 13 is coaxially positioned an elongated capillaryhollow discharge tube 33 held within an inner end 35 of the tubulation21 and at its other end by the inner wall of the anode envelope portion17. The plasma tube 13 also holds an active laser gas or mixture such asNe-Ne, for example. The gas is allowed to circulate within the plasmatube 13, the capillary tube 33 through an aperture 37 in the tubulation21 and the area adjacent an anode terminal 39 in the anode envelopeportion 17. However, the active gas is prevented from propagating to anarea adjacent the second end 25 of the plasma tube 13 by a gascontainment member 41 which may be any laser energy-transmissive windowsuch as the Brewster angle window shown.

The active laser gas may be excited to a lasing state by anyconventional pump means such as a conventional pump source connected toa discharge arrangement including the anode electrode 39 and acylindrical cathode electrode 43 whose outer surface substantiallycoincides with the inner surface of the plasma tube 13. The cathode 43may be fabricated from aluminum or tantalum preferably with a thin oxidelayer on its inner surface. The cathode electrode 43 is connected tocathode-conductive pins 45 of tungsten, for example, by means ofspringlike conductive contactors 47. Although two conductive pins 45 areshown, it is apparent that only one is necessary to provide electricalcontact to the cathode 43. In practice, however, two, three, or evenmore may be desirable for mechanical support. In this regard, referencemay be made to a gas discharge tube disclosed in a copending applicationSer. No. 703,384 which is assigned to the assignee of this invention.

The novel construction technique described above allows the outputmirror 29 to be aligned and permanently attached in place, by an epoxyfor example, and the axially aligned light-transmissive window 41positioned by means of precision mating surfaces of the window and theadjacent inner surface of the anode envelope portion 17, and alsoepoxied in place. The gastight compartment to the left of the window 41is then complete and may be processed to provide the oxide coating onthe cathode electrode 43, for example. After processing, the highlyreflecting adjustable mirror 31 is then aligned with the laser 11operating to the point of attaining maximum power output and then alsopermanently attached in place.

With reference to the embodiment illustrated in FIG. 2, the anodeportion 17 of the plasma tube 13 is provided with a transverselydisposed, nonpolarizing light-transmissive window 51 which serves toprevent the active gas from reaching the area immediately adjacent theadjustable mirror 31. The inner window 41 of FIG. 1 and the inner window51 of FIG. 2 may also serve to provide a filter compartment 53 which maybe filled with a gas or gas mixture adapted to suppress the competitioneffects previously mentioned in connection with the prior art. Forexample, in a 6328" A. He-Ne laser, methane gas may be disposed in thefilter compartment 53 to suppress a competing 3.39;; infrared line. Ithas also been found that the material used for the fabrication of thewindow may itself act as a filter at a desired wavelength. For example,the windows 41 and 51 may be of crown glass or borosilicate glass whichwill suppress the unwanted 3.39u line. Further, the window 51 may bepartially reflective so that it, in conjunction with the adjustablemirror 31, will act as an etalon to provide single line output.

The compartment 53 may also be provided with a communicating tubulation55 as seen in FIG. 2. This will allow the evacuating or thepressurization of the compartment 53 at any time during or subsequent tothe final alignment of the adjustable mirror 31. Thus, this area may befilled with a desired gas to a desired pressure for filter purposes orfor tuning purposes since variations in pressure of the gas within thelaser regenerative cavity tends to, in effect, change the length of theregenerative path and the frequency of laser oscillation.

From the foregoing, it should be evident that the invention provide aunique gas laser that exhibits the same stable, maintenance-free cavityqualities as is available in conventional internal mirror lasers andwherein the laser output power can be maximized under ideal conditions,that is, while the laser is operating. Also, the invention provides anadvantageous means to produce a linearly polarized output beam, and asimple means to suppress undesired competition effects.

In practicing the invention, any active laser gaseous material may beused and any conventional material such as suitable glass, quartz orceramics may be utilized in the fabrication of the plasma and dischargetubes.

It is intended that the foregoing disclosure and drawing shall beconsidered only as illustrations of the principles of this invention andare not to be construed in a limiting sense.

What is claimed is:

1. A gas laser comprising:

an elongated plasma tube having first and second ends;

regenerative cavity means including a fixed mirror at said first end andan adjustable mirror at said second end for defining alaser-regenerative path between said mirrors through said plasma tube;

isolation means disposed in said plasma tube and spaced from saidadjustable mirror for providing a gastight discharge compartment betweenitself and said first end while maintaining said regenerative path, saidisolation means being a light-transmissive window perpendicularlypositioned with respect to said regenerative path and acting inconjunction with said adjustable means as an etalon tuned to a desiredwavelength;

an active laser gas disposed in said discharge compartment of saidplasma tube; and

means coupled to said active laser gas for exciting said gas to a lasingstate.

